This invention relates to an improved process for the preparation of copolymers comprising an aromatic vinyl compound, a vinyl cyanide compound and if desired, other vinyl compounds copolymerizable therewith and a thermoplastic resin composition containing such copolymers.
For improving a molding processability of thermoplastic resin compositions and elevating the thermal deformation temperature, there have, hitherto, been provided processes for the copolymerization of a monomer mixture mainly comprising an aromatic vinyl compound and a vinyl cyanide compound wherein the vinyl cyanide compound and/or aromatic vinyl compound are further added in the latter stage of polymerization (e.g. Japanese Patent Publications No. 45-33661 and No. 47-46472).
However, copolymers obtained by these processes have a wide distribution in the molecular weight and polymer composition, colour upon the molding processing and are insufficient in the impact resistance and heat resistance. Thus, when these copolymers are incorporated into, for example an acrylonitrile-butadiene-styrene terpolymer composition (hereinafter referred to as ABS resin), a thermoplastic resin composition not colouring upon the molding processing cannot be obtained with a good impact resistance and heat resistance.
Particularly, in case copolymers obtained in the process of further adding the vinyl cyanide compound in the latter stage of polymerization are incorporated into the ABS resin, the colouration upon the molding processing is remarkable.